Emily starts counting at 39, and she counts by threes. If 39 is the 1st number that Emily counts. what is the 10th number that she counts?
Solution: What is the first number that she counts? $39$ What is the second number that she counts? $\begin{align*}&39 + 3 \\ &= 42\end{align*}$ What is the third number that she counts? $\begin{align*}&39 + 3 + 3 \\ &= 39 + (2 \times 3) \\ &= 45\end{align*}$ What is the 10th number that she counts? $\begin{align*}&39 + (9\times3) \\ &= 39 + 27 \\ &= 66\end{align*}$